1. Field of the Invention
Embodiments of the present invention relate to an image forming device. Specifically, the embodiments relate to an image forming device including a liquid discharge head that utilizes piezoelectric elements for generating pressure.
2. Description of the Related Art
As an image forming device such as a printer, a facsimile machine, a copier, a plotter, or a combined machine thereof, an image forming device (an ink jet recording device) of a liquid discharge recording type that utilizes a recording head including a liquid discharge head (a liquid droplet discharge head) that discharges an ink droplet is known. The image forming device of the liquid discharge recording type forms an image (recording, typing, imaging, and printing are used as synonyms) by discharging ink droplets from the recording head onto a sheet being conveyed. Here, the sheet is not limited to a paper, but including an OHP and the like. The sheet means something to which an ink droplet, or other liquid can be adhered. It may be referred to as a medium to be recorded on, a recording medium, a recording paper, or a recording sheet. There are two types of the image forming devices of the liquid discharge recording type. Namely, one of them is an image forming device of serial type, in which a recording head forms an image when the recording head moves in the main scanning direction while discharging liquid droplets. The other one is an image forming device of line type which utilizes a line-type head.
In the present specification, the image forming device of liquid discharge recording type is a device which forms an image by discharging a liquid onto a medium, such as a paper, a line, a fiber, a fabric, a leather, a metal, a plastic, a glass, a timber, or a seramic. Further, “forming an image” means not only to add an image having a meaning, such as a character or a graphic, to a medium, but also to add an image having no meaning, such as a pattern, to a medium (simply to apply liquid droplets to the medium). Further, “an ink” means not only a usual ink, but is also a generic term of a liquid with which an image can be formed, such as a recording liquid, a fixing liquid, or a fluid. For example, a DNA sample, a resist, a pattern material, and a resin are included in “inks.” Further, a material of “a sheet” is not limited to a paper, and “a sheet” means something to which ink droplets adhere, including the above described OHP sheet and fabric. Namely, the term “a sheet” is used as a generic term for referring to something to which ink droplets adhere, such as a medium to be recorded, a recording medium, a recording paper, or a recording sheet. Further, “an image” means not only a two-dimensional image, but also an image attached to something which is formed three-dimensionally and an image which is formed three-dimensionally.
As a liquid discharge head, a so-called “piezoelectric type head” is known. Here, the piezoelectric type head includes a piezoelectric body as a pressure generating means that applies pressure to an ink, that is, for example, a liquid inside a liquid chamber. The piezoelectric type head includes, for example, a piezoelectric actuator in which plural pillar-shaped piezoelectric elements (piezoelectric poles) are formed by grooving a laminated piezoelectric member in which piezoelectric layers and internal electrodes are alternately laminated. Alternatively, for example, the piezoelectric type head includes a piezoelectric actuator in which electrodes are arranged to nip a piezoelectric layer and which is formed of a thin-film piezoelectric material. The piezoelectric type head causes an oscillation plate, which can be elastically deformed and which forms a wall surface in the liquid chamber, to be deformed using the piezoelectric actuator, and causes a volume and pressure inside the liquid chamber to vary, and discharges liquid droplets.
As a drive control circuit for driving and controlling such a piezoelectric type head, the following circuit has been known. Namely, the circuit includes a drive waveform generating circuit that generates a common drive waveform in which plural drive pulses are arranged in time series; and a selection unit (driver IC) that selects desired drive pulses from the common drive waveform depending on image data and that applies the selected drive pulses to the corresponding individual piezoelectric elements included in the piezoelectric actuator. In such a case, the common drive waveform and the selected drive pulses are transmitted from the drive waveform generating circuit to the head through a flexible flat cable (FFC). However, the FFC includes resistance components, capacitance components, and inductance components. Further, the main components of the piezoelectric elements included in the piezoelectric type head are the capacitance components.
Therefore, when the number of the simultaneously driven piezoelectric elements is increased, the resonant frequency of a closed loop circuit in the piezoelectric type head is varied. Here, the closed loop circuit starts from the drive waveform generating circuit and ends at the drive waveform generating circuit through the piezoelectric elements. On the other hand, when the drive frequency of the drive waveform coincides with the resonant frequency, as the drive frequency of the head is increased so as to perform high-speed printing, gain of the waveform is increased, and a waveform having amplitudes exceeding desired signal levels is applied to the piezoelectric elements. In such a case, since a discharging speed and a discharging amount of liquid droplets are increased, the sizes of dots to be formed are enlarged. Therefore, there is a problem that density irregularities occur and image quality is lowered.
In regard to the relationship between the drive frequency of the drive waveform and the resonant frequency in the drive circuit, the following technique has conventionally been known. For example, a start-up time of a voltage of a waveform applied to piezoelectric elements is controlled so that the start-up time of the voltage becomes longer than a resonant period specific to the piezoelectric elements (Patent Document 1 (Japanese Published Unexamined Application No. H10-146970)).
Further, in regard to the variation of the drive waveform, the following technique has been known. In the technique, the following circuit is used as a driving circuit for driving plural piezoelectric elements. Namely, the circuit includes sets of three analog switches, the three analog switches being connected in parallel to the corresponding piezoelectric element. Here, the sets of three analog switches are connected in parallel. At every discharging timing, accumulated image data is obtained from image data. The variation of the waveform is suppressed by switching the selected analog switch among the analog switches, depending on the threshold value that has been set (Patent Document 2 (Japanese Published Unexamined Application No. 2008-254204)).
Further, a technique that handles a drive waveform as information about inflection points has been known. In the technique, a waveform input to a piezoelectric element is regulated to be constant by varying the inflection points in accordance with the number of the simultaneously driven piezoelectric elements (Patent Document 3 (Japanese Published Unexamined Application No. 2002-036535)).
However, in the configuration disclosed in Patent Document 1, driving voltage itself may be varied. Thus there is a problem that the control is complicated.
Further, for the configuration disclosed in Patent Document 2, peaking (a phenomenon that signal gain of a drive waveform becomes extremely large) is not considered. Here, peaking is caused by inductance components included in a transmission line connecting a drive waveform generating circuit and a recording head. Therefore, there is a problem that the variation of the waveform associated with an increase of the number of simultaneously driven piezoelectric element may not be suppressed.
Further, with the configuration disclosed in Patent Document 3, the drive waveform may be corrected based on information about the original waveform. However, since the inflection points are varied, a drive waveform that includes correction information and to be output from a drive waveform generating circuit includes more high frequency components, compared to a drive waveform that does not include correction information. Therefore, higher-performance elements may be required, and there is a problem that the cost is increased.
The embodiments of the present invention have been developed in view of the above described problems. An objective of the embodiments is to improve image quality by reducing variation of a drive waveform caused by variation of the number of simultaneously driven piezoelectric materials, and by reducing variation of discharging characteristic.